Spine

CERVICAL SPINE

Discuss the anatomy of the atlas (C1 cervical vertebra) including the associated joints and ligaments (30 marks)

PARTS

• Anterior arch
• Paired lateral masses with their superior and inferior articular facets
• Bilateral large transverse processes with transverse foramen for vertebral arteries
• Posterior arch
• (No body – replaced by dens of C2)
• Two parts of vertebral canal: anterior 1/3 = dens, posterior 2/3 spinal cord

Anterior arch

• anterior tubercle: sits on the anterior aspect of the anterior arch and is the site of attachment of the anterior longitudinal ligament
• posterior facet for the dens: sits on the posterior aspect of the anterior arch
• upper border: attachment of the anterior atlanto-occipital membrane and lateral parts of the anterior longitudinal ligament
• lower border: attachment of the anterior atlanto-axial membrane and lateral parts of the anterior longitudinal ligament

Posterior arch

• 3/5th of circumference of the ring
• posterior tubercle: sits posteriorly to the posterior arch, is a rudimentary spinous process and attachment site for the ligamentum nuchae
• superior surface: contains paired grooves for the C1 nerve and vertebral artery, sits just posterior to the lateral mass
• superior border: attachment for the posterior atlanto-occipital membrane
• inferior border: attachment for the ligamentum flava

Lateral masses

• paired, ovoid
• superior articular facet: kidney-shaped, concave and articulates with the occipital bone
• inferior articular facet: circular, with a flat or slightly concave surface articulating with the lateral atlanto-axial joint
• medial surface: marked by vascular foramina and a tubercle for the attachment of the transverse ligament

Transverse processes

• longer than all of the transverse processes of the cervical vertebrae except C7
• typically covered by costal lamella
• transverse foramina: contains the vertebral arteries
• anterior tubercle: sometimes present on the anterior aspect of the transverse process

ACTION

• Atlantooccipital joint: 50% cervical flexion/extension and limited lateral motion
• Atlantoaxial joint: 50% cervical rotation

JOINTS

Atlantooccipital Joint (atlas & occipital)

• Inferior articular facet of occipital condyle: Oval, convex surface, projects laterally
• Superior articular facet of C1: Oval, concave anteroposteriorly, projects medially
  • kidney shaped concave facets to cup occipital condyles
  • allows nutation (anterior posterior flexion)
  • capsule is thin and loose

Median atlantoaxial joints = 2 joint cavities – anterior and posterior (atlas & dens of C2)

• Pivot type joint between dens with ring formed by anterior arch with transverse ligament of C1
• 2x Synovial cavities between transverse ligament/odontoid and atlas/odontoid articulations
  • Anterior median atlantoaxial joint – posterior surface of anterior arch concave, almost circular facet for dens
  • no capsule

Lateral atlantoaxial joints (atlas & axis (C1 to C2))

• Inferior articular facet of C1: Concave, projects medially in coronal plane
• Superior articular facet of C2: Convex surface, projects laterally
• allows nutation (nodding)

*Tip: C1 superior and inferior articulations always concave + projects medially!

LIGAMENTS

From anterior to posterior: anterior atlantooccipital membrane —> odontoid ligament (apical & alar ligament) —> Cruciate ligament (transverse + craniocaudal) —> Tectorial membrane —> Posterior Atlantooccipital membrane —> Ligamentum nuchae

• Anterior atlantooccipital membrane
  • Connects anterior arch of C1 with anterior outer margin of foramen magnum.
  • Continues inferiorly (from median cord) as anterior longitudinal ligament
• Odontoid ligaments
  • Apical ligament: Small fibrous band extending from dens tip to basion
  • Alar ligaments: Thick, horizontally directed ligaments extending from lateral surface of dens tip to anteromedial occipital condyles
• Cruciate ligament
  • Transverse ligament: Strong horizontal component between lateral masses of C1, passes behind dens
  • Craniocaudal component: Fibrous band running from transverse ligament superiorly to foramen magnum and inferiorly to C2
• Tectorial membrane:
  • Continuation of posterior longitudinal ligament; attaches to anterior rim of foramen magnum (posterior clivus)
• Posterior atlantooccipital membrane
  • Posterior arch C1 to posterior margin of foramen magnum
  • Deficit laterally to allow C1 nerve passage and vertebral artery enters on superior surface of C1 (variant – if lateral opening calcifies “arcuate foramen”)
  • Innervated by C1
• Ligamentum nuchae
  • Attaches to the posterior tubercle which is a rudimentary spinal process
  • Attaches to spinous process of cervical vertebrae and occiput
  • Limits forward lesion

MUSCULAR ATTACHMENTS to C1

• Anterior:
  • Longus colli – superior oblique parts attaches to anterior surface of C1 body (anterior)
  • Rectus capitus anterior – attaches to anterior surface of the lateral mass of C1 (anterior)
• Lateral:
  • Rectus capitus laterus – attached to transverse process of C1 superiorly – up to skull (lateral)
  • levator scapulae – tip of transverse process of C1 (lateral)
• Posterior:
  • Rectus capitus posterior minor – attaches to posterior tubercle of C1 arch (Posterior)
  • splenius cervicis – transverse process of C1 (posterior)
  • Obliquus capitus superior – attached to transverse process of C1 (posterior)
  • Obliquus capitus inferior – attached to apex transverse process of C1 (posterior)

VARIANTS

• unfused arch of atlas : anterior, posterior, both
  • central part of the posterior arch can be absent and replaced by fibrous tissue
• arcuate foramen: calcification of the posterior atlanto-occipital membrane
• superior articular facets of the lateral masses can be divided into two parts, with the anterior part being larger and the posterior smaller
• fusion (with the occiput – partially or fully)
• accessory ossicle of the anterior arch of the atlas